1. Field of the Invention
The present invention relates to a control apparatus of a support unit for optimumly controlling characteristics of the support unit, that is, a suspension and/or a stabilizer, that supports a vehicle body corresponding to vibrations thereof.
2. Description of Related Art
It is better, for improving comfort to ride in while a vehicle is running, to adapt characteristics of a support unit of the vehicle body to be SOFT. Specifically, it is better that a damping force of a suspension is small, a spring constant thereof is small and soft, and a torsion quantity of a stabilizer is small. On the other hand, it is better, for improving steering stability, to adapt the characteristics of the support unit of the vehicle body to be HARD. Specifically, it is better to increase the damping force of the suspension, to make large and hard the spring constant thereof, and to increase the torsion quantity of the stabilizer.
A control apparatus, which automatically controls to an optimum value the damping force or spring constant of the suspension corresponding to the vertical vibration of the vehicle body, is disclosed in the Japanese Patent Application Laid-Open No. 61-18351. The control apparatus detects vertical vibrations so that, when the detected vertical vibrations exceeds a predetermined amplitude and has a predetermined frequency, it is judged that the vertical vibrations continue, whereby the damping force or spring constant of the suspension is changed over to be larger to restrain continuous vibrations during the vehicle's running on an undulated road.
The vertical vibrations of the vehicle body, detected by an acceleration sensor during the running, changes as shown in, for example, FIG. 5 or 7. Generally, a waveform of low frequency, and large amplitude correspond to above-spring vibration, and a waveform of high frequency and small amplitude correspond to below-spring vibration. The output waveform of the acceleration sensor is periodically sensed to determine the presence of either above-spring vibrations or below-spring vibrations, thereby restraining variations in the posture of the vehicle body on the basis of the detection result.
However, when the detected vibration has a low frequency, a decision time is longer creating a defect, by delaying proper restraint of the variation in the posture of the vehicle body. When a sampling time for an output signal of the acceleration sensor is set (for example, to 8 msec) and the detected vibration has a high frequency, if the decision time is assumed to be a half cycle period, resolution of sampling is poor, so that an error is created more frequently in the determination of frequency as the vibration is higher in frequency. Furthermore, the periodical sensing of the output waveform of the acceleration sensor is defective in that it is difficult to distinguish the above-spring vibration from the below-spring vibration.